(1) Field of the Invention
This invention relates to an Al-Sn base bearing alloy which is made by adding tin to an aluminum matrix and relates to a bearing material which is made by applying the above bearing alloy to a backing steel sheet by pressure welding. More paticularly, the invention relates to an aluminum base bearing alloy which is low in the growth of tin particles and in the lowering of hardness, and is excellent in fatigue strength at high temperature conditions. The Al-Sn base bearing alloy of the invention is suitable for use after several repetitions of rolling and annealing subsequent to casting.
(2) Description of the Prior Art
The Al-Sn base alloys are mainly used as aluminum base bearing alloys in the conventional art. When these conventional alloys are used for the bearings of automobile internal combustion engines, fatigue failure is sometimes caused to occur in a short time if the engines are continuously operated under heavy loads. This is considered to be due to the fact that the temperature of lubricant oil in an internal combustion engine becomes very high during the continuous full-load running, for example, the temperature of the lubricant oil in an oil pan reaches 130.degree. C. to 150.degree. C., so that the temperature of the sliding surface of the bearing is also raised very high. As the result, since the eutectic point of such the alloy is about 225.degree. C. or so, the hardness of the alloy rapidly becomes low under the high temperature conditions, which causes the fusion and the migration of the Sn component and the fatigue strength is resultantly lowered. The inventors of the present invention have prepared an alloy, the hardness of which is not lowered at high temperatures and the tin component of the alloy is hardly movable. The alloy was worked into the shapes of bearings for internal combustion engines and they were subjected to fatigue tests under dynamic loads at high oil temperatures. As a result, the improvement in fatigue strength was recognised, which substantiated the above-mentioned consideration.
Further, in addition to the lowering of the fatigue strength due to the loss of hardness at high temperatures as mentioned above, the coarsening of tin particles in the texture of a conventional Al-Sn base alloy also causes lowering of the fatigue strength. That is, the aluminum bearing material is generally formed by applying the Al-Sn base alloy to a backing steel sheet through pressure welding, in which an annealing step is required after the pressure welding in order to improve the adhesive strength between both metals. The annealing is generally done at a temperature below the point (about 475.degree. C.) at which an Al-Fe inter-metallic compound deposits and the higher the treating temperature is and the longer the treating time is, the larger the adhesion strength becomes. As a matter of fact, when the conventional Al-Sn alloy is placed in a high temperature condition during annealing, the migration of aluminum grain boundaries and tin particles is caused to occur in the alloy texture and, as a result, the tin particles become coarse. That is, when the conventional aluminum bearing alloy is subjected to annealing in order to improve the adhesive strength to the backing steel, the coarsening of tin particles is brought about, which results in the lowering of the fatigue strength of the Al-Sn base alloy at high temperatures.